Poly(hydroxystyrene) (PHS) is an excellent resin for use in photoresists and other photosensitive imaging systems. It is particularly useful in the deep ultraviolet region of the spectrum because of its low absorption of light in that region compared with the commonly used novolac resins. PHS resins in which some or all of the phenolic groups are functionalized with t-butyloxycarbonyl ("t-Boc") have advantages over non-functionalized PHS. The t-Boc groups slow the rate of dissolution in base, but are readily removed by traces of acid, which causes decomposition of the t-Boc group to isobutylene and CO.sub.2, as illustrated below for the t-Boc derivatized poly(4-hydroxystyrene): ##STR1##
The t-Boc functionalized PHS resins are particularly useful in positive photoresists in which a light sensitive compound is included which releases a trace of acid upon exposure to light. Areas of the resin coating which have been exposed to light dissolve more rapidly in basic developer solutions because of the larger concentration of free phenolic groups. Such resins give improved contrast over novolac resins.
The t-Boc PHS resins are generally made by functionalizing the hydroxyl groups of the preformed PHS polymer with di-t-butyldicarbonate in the presence of a base catalyst. Generally, only a fraction of the hydroxyl groups are converted to t-Boc groups, depending on the desired dissolution characteristics. The functionalized resin is generally precipitated with water, filtered, and dried in a vacuum oven. It is then stored in the dry state until it is to be used in a photoresist composition, at which time it is dissolved in a solvent suitable for coating onto a substrate.
The solid functionalized PHS resin is hard to dry thoroughly because the wet solid hydrolyzes slowly at room temperature and faster when it is warmed. The resin is also hydroscopic. As a result, there are batch to batch variations in the amount of t-Boc functionality in the freshly synthesized polymer. Furthermore, the amount of t-Boc functionality changes with time during storage because the resin in powder form is generally not completely dry. These variations in the level of t-Boc functionality are unpredictable. Because the dissolution rate of coatings made from the functionalized resin varies according to the amount of free hydroxyl in the resin, these variations significantly complicate the use of these resins in photolithography.